RAPID AMPLIFICATION OF A RETROTRANSPOSON SUBFAMILY IS EVOLVING THE MOUSE GENOME

Retrotransposition affects genome structure by increasing repetition a nd producing insertional mutations(1,2). Dispersion of the retrotransp oson L1 throughout mammalian genomes suggests that L1 activity might b e an important evolutionary force(1). Here we report that L1 retrotran sposition contributes to rapid genome evolution in the mouse, because a number of L1 sequences from the T-F subfamily are retrotransposition competent. We show that the T-F subfamily is large, young and expandi ng, containing approximately 4,800 full-length members in strain 129. Eleven randomly isolated, full-length T-F elements averaged 99.8% sequ ence identity to each other, and seven of these retrotransposed in cul tured cells. Thus, we estimate that the mouse genome contains approxim ately 3,000 active T-F elements, 75 times the estimated number of acti ve human L1s. Moreover, as T-F elements are polymorphic among closely related mice, they have retrotransposed recently, implying rapid ampli fication of the subfamily to yield genomes with different patterns of interspersed repetition. Our data show that mice and humans differ con siderably in the number of active L1s, and probably differ in the cont ribution of retrotransposition to ongoing sequence evolution.